Apparatus for electrochemical removal of heavy metals such as chromium from dilute wastewater streams using flow-through porous electrodes

ABSTRACT

A stream containing chromium and other heavy metals is fed through an electrolytic cell having a flow-through anode of lead shot and a flow-through cathode. The stream passes through the lead shot, resulting in the formation of lead chromate at the anode that falls to a trap in the bottom of the cell. Heavy metals such as copper are plated out on the material forming the flow-through cathode.

BACKGROUND OF THE INVENTION

The invention relates to processes for purifying industrial wastestreams and more particularly to processes for treating waste solutionselectrochemically to remove chromium and other heavy metal ions.

Chromium-containing solutions are used in a number of industries usuallyfor chrome plating steels or for inhibiting corrosion in steel vessels.These solutions are acid solutions having various compositions dependingupon the article to be treated. Hexavalent chromium in some form--suchas sodium dichromate, sodium chromate, potassium dichromate, potassiumchromate, chromium tri-oxide (chromic acid) and the like--is present inthese solutions. Because hexavalent chromium is one of the most toxicchemicals to fish life, in even very minute concentrations on the orderof one part per ten million, it must be substantially completely removedbefore treating solution waste waters containing chromium are dischargedin a sewage system. Other metals and metal compounds which are found insuch treating solutions can be precipitated out by suitably adjustingthe pH of the waste waters. Hexavalent chromium, however is soluble at apH of 0-14 and therefore special methods must be employed to remove it.

There have been various methods proposed to capture chromium beforepassing waste into a water body or sewer system. For example, U.S. Pat.No. 3,494,328 discloses metering stoichiometric amounts of a leadcompound (such as lead nitrate) into a treating bath containing chromateions to form an insoluble lead chromate. However, as is reported in U.S.Pat. No. 3,791,520, water containing a few parts per million hexavalentchromium was run through a bed containing a particulate insoluble leadcompound such as lead oxide or hydroxide. Flow through the bed was veryslow due to the formation of insoluble lead chromate which tended tocause packing of the bed. Channeling of the bed also occurred withupward flow of water therethrough with the result that the chromium inthe water was not adequately removed.

U.S. Pat. No. 3,791,520 discloses contacting the chromium-laden waterwith a relatively water insoluble lead compound in conjunction with aporous, particulate matrix which prevents packing and channeling of theinsoluble lead compound. The effluent from which the chromium has beenremoved is then run through a cation exchanger in the hydrogen form toremove any solubilized lead ion.

Removal of chromium from rinse waters by ion exchange methods is wellknown but is less than satisfactory because of the cost, the fact thatthe life of the ion exchange resin is short and can only be regenerateda few times before replacement, and because regeneration of the ionexchange resin yields a concentrated chromium solution which stillpresents a disposal problem.

A well known and commonly used method of removing chromium is to reducechromium from the hexavalent state to the trivalent state with SO₂ andthen adjust the pH with calcium hydroxide to precipitate out thetrivalent chromium together with relatively large quantities of calciumsulfate (gypsum) sludge. This method suffers from two seriousdeficiencies: (1) the gypsum has little commercial utility, and (2) theamount of trivalent chromium is usually too small and too contaminatedby the gypsum to be of any commercial value. A further disadvantage ofthe SO₂ method is that no free water is obtained that can be recycledfor reuse in the metal treating system. Other methods of reducinghexavalent chromium employ sugar, wood, molasses and sawdust and arelikewise unsatisfactory because of the production of large amounts ofsludge having little or no commercial utility.

With regard to the removal of other metals, electro-chemical designshave been tested for nickel or zinc removal in dilute acidic or neutralsolutions. In H.S.A. Reactors Ltd., "Metal Finishing Report," November1977, pp. 26, H.S.A. Reactors Ltd. reported nickel reduction from 132ppm to 14.5-22.0 ppm using a carbon fiber matrix cathode. This procedurehas been termed "direct electrowinning." A fluidized bed "Chemelec" cellas reported in C. L. Lopez-Cacicedo, "The Recovery of Metals from RinseWaters in `Chemelec` Electrolytic Cells," Trans. Inst. Metal Finishing,53, 1975, pp. 74-77, reduced zinc from 523-550 ppm to 333-427 ppm andattempts at nickel reduction were simply termed "inconclusive." Bycontrast the "Swiss-roll" as reported in P. M. Robertson and N. Ibl,"Electrolytic Recovery of Metals from Waste Waters with the `Swiss-roll`Cell," J. Appl. Electrochem., 7, 1977, pp. 323-330, was used toselectively recover copper from copper/zinc and copper/nickel mixtures.It was reported that 99.9% of the copper was recovered with nodetectable change in the concentration of the second metal.

These results show that complete removal of copper from acidic solutionsis possible and significant nickel and zinc removal from dilute acidicsolutions is possible when a flow-through cathode is used.

In accordance with the present invention, the direct electrowinningprocess may be used for the removal of copper and other heavy metals atthe cathode and chromate removal at the anode.

Accordingly an object of this invention is to provide an electrochemicalmethod and apparatus for the efficient removal of chromium and otherheavy metals from dilute solutions.

SUMMARY OF THE INVENTION

Heavy metals are removed from dilute solutions in an electrolytic cellcontaining an anode of lead shot in a packed bed. Any flow-throughcathode may be used. As the solution passes through the sacrificial leadanode, lead chromate is formed electrochemically in tiny particles whichslough off and settle in a trap at the bottom of the cell. Other heavymetals plate out at the cathode.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic view of an electrolytic cell in accordance withthe present invention;

FIG. 2 is a diagrammatic view of a four-stage cell used to conduct testson the electrodeposition of non-chromium metals; and

FIG. 3 is a diagrammatic view of a cylindrical cell in accordance withthe present invention in which current is perpendicular to solutionflow.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an electrolytic cell 10 made up of two electrodes 12 and 14in a container 15 for holding electrolyte (waste water) 17. There is aninlet tube 18 feeding liquid through an opening in the upper part ofcell container wall 15 at one end of the cell, and an outlet tube 20connected to an opening in the upper part of a cell container wall atthe opposite end of the cell.

One of the electrodes 12 in the cell, near the inlet end of the cell, isan anode of lead shot in the form of a packed bed. The other electrode14, near the outlet end of the cell, is a cathode of graphite granulesor other carbonaceous conductive particles in the form of a packed bed.Each electrode is attached to a terminal, the anode to a positivevoltage and the cathode to a negative voltage.

The bottom wall of the cell container includes a trap 16, a cavityformed to be below the surface on which the electrodes 12 and 14 rest.The trap occupies a major portion of the bottom of the cell between theelectrodes.

In use, waste water containing chromate and other heavy metal ions isfed to the cell through the inlet tube 18. Voltage is applied to theelectrodes. Effluent is withdrawn through the outlet tube 20.

The chromate ion in solution combines with the lead in the anode to formlead chromate. The anodic reaction is thus:

    Pb+CrO.sub.4.sup.-2 →PbCrO.sub.4 ↓+2e-

The cathodic reaction is the deposition of a metal such as Cu, Ni or Zn.The lead chromate is formed as tiny particles that settle in the trap atthe bottom of the cell, from which they may be removed.

In a test of a cell like that shown in FIG. 1, a feed solutioncontaining 200 ppm (parts per million) each of Cu⁺², Ni⁺², Zn⁺², andCr⁺⁶, was run through the cell. The voltage across the electrodes was 8volts. After several hours, steady state conditions were reached, givingthe effluent concentrations below for two experiments:

    ______________________________________                                        Flow Rate                                                                             Cell     Effluent Concentration (ppm)                                 (ml/min.)                                                                             Amps     Cu     Ni   Zn   Cr.sup.+6                                                                          Cr Total                                                                             Pb                              ______________________________________                                        2       0.05     <0.5   30   35   <0.5 <0.5   50                              8       0.09     90     165  160  <0.5 30      1                              ______________________________________                                    

Lead went into solution only after complete removal of Cr⁺⁶, but waspartly removed by reduction at the cathode. It was found that leadchromate (PbCrO₄) was not produced from solutions containing only Cr⁺⁶,but that other metals (Cu⁺², Ni⁺², Zn⁺², or Na⁺) were required for somekind of catalytic action. Thus, an aspect of the invention is to includea cation to promote the reaction of lead chromate if other cations arenot present in the waste stream. If heavy metals such as copper, nickel,cobalt, zinc, silver, gold, cadmium, etc. are not present along with thechromate ion, a cation is actually added to promote lead chromateformation. A preferred cation for addition to the waste water is sodiumbecause it is not expensive and need not be removed from solution.

Solutions were prepared with analytical reagent-grade chemicals ofcupric nitrate, nickelous nitrate, zinc sulfate and technical-gradechromic acid (CrO₃) to produce a starting solution of 200 ppm each ofCu⁺², Ni⁺², and Cr⁺⁶ on which to test the process and apparatus of theinvention. Such solutions have a natural pH of 2.3 (the acidity duealmost entirely to chromic acid) and a conductivity of ˜3×10⁻³ (ohmcm)⁻¹.

Two kinds of particulate graphite were used. The first from AsburyGraphite Mills, Inc. (grade 4228) containing 99.4% C, the mainimpurities being SiO₄, CaO, S, Fe₂ O₃ and Al₂ O₃ in decreasing order.The second was a high-purity graphite from Ultra Carbon Corp. whichcontained only 2.5 ppm total metallics. Size fractions of 10×20 mesh or10×40 mesh were used for all tests.

The lead shot was of technical grade which had not been analyzed forimpurities. Only in the cylindrical cell of FIG. 3 was 1/8" diametershot used. All other tests used 1/16" diameter shot.

The basic cell as shown in FIG. 1 consisted of an open-top Plexiglasscube approximately 4" on a side. A packed graphite bed acted as acathode and was supported by a perforated plate or screen 24 (stainlesssteel, lead-antimony, graphite which functioned as the current collectorand was attached to the negative terminal) and a plastic screen 26.Similar screens 24' and 26' supported the lead shot anode. Thus, plate24' functioned as the anode current collector and was attached to thepositive terminal. Screens 26, 26' and frames 24, 24' containing thepacked beds provided an exposed superficial area of 52 cm² to the waterline for each bed. The submerged bed volume was 140 cm³ for each bed.Variations of this design are mentioned below as they apply to specifictests.

The second cell design, used to investigate reactions in multiplestages, is shown on FIG. 2. As in the single-stage cell of FIG. 1,solution flow and current were in the same direction (parallelgeometry). No chromium solutions were used when operating this cell sothat lead shot anodes were unnecessary and were substituted by single,perforated graphite plates as anodes 30.

The third cell design consisted of two concentric cylinders as shown inFIG. 3. The inner cylinder 32, a porous tube of Vycor (Corning GlassCo.) or ceramic (Coors Porcelain Co.) allowed electrical contact withoutmixing of anolyte and catholyte. A Plexiglas cylinder 34 formed thehousing for the cell. The annular space 36 between housing 34 and porouscylinder 32 was filled with conductive particles 38 such as graphitechips. Two or more symetrically spaced negative current collectors(graphite rods) 40 were positioned within the annular space 36 inelectrical contact with chips 38. A pump 42 and a conduit 44 deliveredchromate free solution from a reservoir 46 to the annular space 36. Theporous tube 32 was filled with lead shot 48 and was in electricalcontact with a positive current collector (graphite rod) 50.

In operation, solution to be treated is delivered through the wastestream feed to the anode (bed or lead shot) 48. With the current on,lead chromate forms on the lead shot and is carried to the bottom of thecylinder 32 with the flow of the waste stream feed. A grid 52 maintainsthe bed of lead shot but allows passage of the lead chromate. Othermetals plate out on the flow through cathode (chips 38), which can beremoved for metal recovery. The cylindrical configuration operates withsolution flow at right angles to current (perpendicular geometry).

Cells designed with "parallel" geometry are simpler to engineer andeasier to handle physically, but suffer from the fact that most of thereaction takes place within the first 1/8" of the bed where the solutionpotential is highest. Solution past the "reaction zone" will react verylittle, and a minimum obtainable metal concentration may not berealized.

Alternatively if the direction of fluid flow is perpendicular to thecurrent, a cell results such that the reaction zone is thin, as seen bythe counter electrode (giving a high and relatively uniform solutionpotential) and long, as seen by the flowing fluid (giving an adequateresidence time). The cylindrical cell of FIG. 3 has this type of"perpendicular" geometry.

The complexity of electrochemical reactions and reactors lead to a longlist of variables, some of which are interrelated:

    ______________________________________                                        Cell voltage       Solution additives                                         Cell current       pH                                                         Current density    Solution conductivity                                      Electrode potential                                                                              Cell geometry                                              Flow rate          Electrode materials                                        Flow direction     Particle size                                              ______________________________________                                    

For packed-bed, parallel geometry reactors, literature indicates thatthe variables in the left column are the most critical. Also, for agiven cell and solution under steady state conditions, the first fourvariables are completely dependent on each other and can be fixed andcontrolled by holding any one constant. Therefore, the first set ofmeaningful experiments was run to test the influence of cell voltage,flow rate and flow direction on metal removal from solution. Eight testswere run in factorial design fashion and are described more fully belowby the following non-limiting examples.

Results of these initial tests showed essentially complete removal ofcopper and chromium.

Test 1-15

During these tests it was established that chromium formed an insolublePbCrO₄ at the lead anode.

Tests 16-23

These eight tests were designed factorially to show the effects of cellvoltage, flow rate and flow direction. Test conditions were as follows:

    ______________________________________                                        Cell design:   cf. FIG. 1; 1-inch space between                                              anode and cathode; 3/4-inch plexi-                                            glass plug in sections 22 and 23                                              to reduce residence time.                                      Solution:      200 ppm each Cu.sup.+2, Ni.sup.+2, Zn.sup.+2,                                 Cr.sup.+6 ; H.sub.2 SO.sub.4 added to adjust pH                               from 2.3 to 2.0.                                               Anode:         Pb shot; 1/16-inch diameter                                    Cathode:       Asbury #4228 (10 × 20 mesh)                              Cell voltage:  2 and 8 volts                                                  Flow rate:     2 and 8 ml/min                                                 Flow Direction:                                                                              anode-to-cathode (A → C) and                                           cathode-to-anode (C → A)                                ______________________________________                                    

In this cell, 8 volts is just below the point where copious amounts ofhydrogen gas were evolved.

Significant findings of this test series were:

1. Cr⁺⁶ was effectively removed from solution (<1 ppm) at the anode byprecipitation of PbCrO₄. Once the solution was depleted of Cr⁺⁶, leadwas solubilized and in some cases, reached concentrations >200 ppm.

2. Low flow and high voltage were more effective in metal removal butcommercially would be more expensive in terms of power cost andequipment sizing. Higher voltage also tended to solubilize more lead.

3. The response of Cr⁺⁶ removal was unexpectedly faster at the lowvoltage. Anodic passivation of lead is known to occur and could explainthe slower kinetics at the high voltage.

4. A→C flow direction was much more effective in metal removal than C→A.An added advantage was that most of the lead which was solubilized atthe anode was subsequently reduced at the cathode.

5. In two tests, Cu⁺² effluents were reduced to <0.5 ppm. Ni⁺² and Zn⁺²could be reduced to <100 ppm only under conditions in which leadconcentration became appreciable.

Tests 24-27

These tests, in a slightly modified cell and fine sized high-puritygraphite, extended the operating range of variables from the previousset of tests. Also more attention was given to quantifying compositionsof solid products and final effluents.

Test conditions were as follows:

    ______________________________________                                        Cell design: cf. FIG. 1; 1/4-inch space between                                            anode and cathode; 3/4-inch plexi-                                            glass plug in sections 22 and 23                                              to reduce residence time.                                        Solution:    200 ppm each Cu.sup.+2, Ni.sup.+2, Zn.sup.+2,                                 Cr.sup.+6 ; H.sub.2 SO.sub.4 added to adjust pH                               from 2.3 to 2.0; SO.sub.4 = concentration                                     707-757 ppm.                                                     Anode:       Pb shot; 1/16-inch diameter                                      Cathode:     Ultra Carbon EX-105 (10 × 40 mesh)                         Flow direction:                                                                            A → C                                                     Cell voltage:                                                                              2 and 10 volts                                                   Flow rate:   3 and 12 ml/min.                                                 ______________________________________                                    

This higher voltage and shorter A-C spacing gave a low enough cathodepotential such that hydrogen evolution was very evident at the stronglyreducing cathode face.

Steady-state conditions and results are given in Table IV.

                  TABLE IV                                                        ______________________________________                                         Conditions and Results of Tests 24-27                                        ______________________________________                                                Cell      Cell      Flow    Current                                   Test    Voltage   Current   Rate    Efficiency                                #       (V)       (amps)    (ml/min)                                                                              (%)                                       ______________________________________                                        24      10        0.16      12      133                                       25      10        0.10       3      144                                       26       2        0.042     12      260                                       27       2        0.030      3      147                                       ______________________________________                                        Effluent Concentration (ppm)                                                                              Effluent                                          Cu   Ni     Zn      Cr.sup.+6                                                                           Cr.sub.T                                                                           Pb    SO.sub.4                                                                            pH                                 ______________________________________                                        120  140    140     18    52   <0.5  282   3.6                                 2    15     14     <1     2    52.0 <25   5.4                                170  195    190     27    90   <0.5  523   3.2                                100  200    195     <1    90    6.0  624   2.5                                ______________________________________                                    

Only in test 25 was the current high enough to remove theoretically allmetals in solution assuming the formation of PbCrO₄. However, the solidsanalysis showed that little, if any, PbCrO₄ was formed but rather thatCr(OH)₃ and hydrolysis products of Cu, Ni and Zn precipitated at thehigher pH. This result confirms previous observations that high voltagesappear to inhibit PbCrO₄ formation.

It should be noted that x-ray analysis confirmed the presence of PbSO₄as well as PbCrO₄. These two compounds account for nearly all leadpresent in the solids except test 25 where some hydroxide is suspected.Hydroxides in general were often noticed on the face of the cathode bedwhere hydrogen gas evolution apparently depleted the solution of enoughhydrogen ions to precipitate hydroxides in localized regions of highpH--higher than observed in the effluent.

The fact that all current efficiencies are greater than 100% showed thathydrolysis, scavenging by PbCrO₄, adsorption by graphite or some othernon-electrochemical form of metal removal was operative. An adsorptiontest was done with a sample of Asbury #4228 using the standard 200 ppmsolution. After three days a sample was taken and stripped with asolution of H₂ SO₄ at pH 0.4. The results, Table VI, show that, with thepossible exception of chromium (CrO₄ ⁻²), adsorption would not bedetectable within the normal scatter of experimental error.

                  TABLE VI                                                        ______________________________________                                        Adsorption Test on Asbury #4228 Graphite                                              Chemical Analysis (ppm)                                               Graphite  Cu        Ni       Zn     Cr.sub.T                                  ______________________________________                                        Initial   5.7       3.4      2.3     5                                        Loaded    13.3      5.6      3.8    45                                        Difference                                                                              7.7       2.1      1.5    40-45                                     Stripped  1.2       2.9      1.8    13                                        ______________________________________                                    

Tests 28-31

These tests were intended to demonstrate chromium removal from solutionscontaining 200 ppm Cr⁺⁶ only. It was found that PbCrO₄ was not formed atall presumably due to a corrosion inhibiting or passivating effect ofchromate ion on the lead surface. However, when drops of solutioncontaining Cu, Ni, Zn or Na were added to the top of the anode bed,giving an overall solution concentration of about 50 ppm, PbCrO₄formation was immediate. Since industrial chrome plating baths containno other metallic cations, the above observation indicates that chromiumwaste streams must be combined with other metalcontaining streams forsuccessful chromate removal.

Tests 32-34

These tests were designed to study the removal of Cu, Ni and Zn in amulti-stage reactor. Unlike previous tests where cell voltage was fixed,these tests were run at constant current noting that the theoreticalcurrent required for complete cathodic removal of metals was 0.93 amps.Test conditions were as follows:

    ______________________________________                                        Cell design: cf. FIG. 2                                                       Solutions:   200 ppm each Cu.sup.+2, Ni.sup.+2, Zn.sup.+2 ;                                pH 5.3                                                           Anodes:      Perforated graphite plates                                       Cathodes:    UltraCarbon Ex-105 (10 × 40 mesh)                          Flow rate:   30 ml/min                                                        Cell Current:                                                                              1, 2 & 3 amps                                                    ______________________________________                                    

Steady state conditions and results are given in Table VII.

                  TABLE VII                                                       ______________________________________                                        Conditions and Results of Tests 32-34                                              Cell     Cell     Current Effluent Conc.                                                                           Eff-                                Test Current  Voltage  Efficiency                                                                            (ppm)      luent                               #    (amps)   (V)      (%)     Cu   Ni   Zn   pH                              ______________________________________                                        32   1.0      3.0      35      <0.5 190  180  2.4                             33   2.0      5.0      26      <0.5 129  128  2.4                             34   3.0      7.5      21      <0.5  93   91  2.3                             ______________________________________                                    

These results show the relative ease of copper removal and thedifficulty of nickel and zinc removal. Samples at each stage showed thatnickel and zinc concentrations decreased after each successive stage butto a lesser degree implying that a large number of stages would berequired to get below the 1 ppm level.

It was observed in the case of copper, that precipitation occurred allthe way through the third cathode bed, but only about 1/8-inchpenetration on the other cathodes. Also precipitation appeared heavierat the bottom than at the top.

Certain hydrolysis products were also noted in tests 33 and 34: a bluishprecipitate in the front end (high pH region) and a greenish precipitatein the back end (low pH region). The former was identified by x-raydiffraction principally as Cu(OH)₂ with some NiOOH. Note that nickel wasoxidized to the +3 state. A third near-amorphous phase was also presentwhich could not be identified. Hydroxides again were formed in localizedregions of high pH even though the equivalent production of anodicoxygen was greater than cathodic hydrogen resulting in a net drop inaverage pH from 5.3 to about 2.4.

Some breakdown of the anode plate was noted in test 34 where thesolution became cloudy black with near-colloidal particles of carbon.

Tests 35-36

These were exploratory experiments intended to test the materials and"perpendicular geometry" concept of the cylindrical reactor, FIG. 3.Test conditions were as follows:

    ______________________________________                                        Cell design:  cf. FIG. 3                                                      Solutions:    200 ppm each Cu.sup.+2, Ni.sup.+2, Zn.sup.+2,                                 Cr.sup.+6 ; pH 2.3                                              Inner tube:   Test 35: Porous Vycor                                                         Test 36: Porous alumina                                         Anode:        Pb shot; 1/8-inch diameter                                      Cathode:      Asbury #4228 (10 × 40 mesh)                               Flow rate:    30 ml/min                                                       Cell current: 1 amp                                                           ______________________________________                                    

In test 35 after about 2 hours operation, effluent concentrationsreached a minimum of 0.5 ppm Cr⁺⁶, 30 ppm Cu⁺², 145 ppm Ni⁺² and 135 ppmZn⁺², after which all concentrations began to rise. After disassemblingthe cell, it was found that the Vycor tube cracked.

Test 36 with the porous alumina tube was more successful in holding upphysically with no apparent cracking or decrepitation. Effluentconcentrations reached a minimum at about 3 hours of <0.1 ppm Cr⁺⁶, 30ppm Cu⁺², 120 ppm Ni⁺² and 110 ppm Zn⁺² after which they began to rise.It was noted that interstices between the #4 lead shot eventually filledwith PbCrO₄ raising the voltage from 7 to 20 volts and inhibiting bothsolution flow and the anodic reaction. Larger balls of shot wouldrelieve these problems.

ELECTROCHEMICAL THEORY Voltage Reguirements

When any electrochemical whole-cell reaction takes place the totalvoltage (V_(T)) measured across the operating reactor is actually thesum of several components.

    V.sub.T =φ.sub.cell +|η.sub.c |+η.sub.a +IR (1)

where

φ_(cell) =electromotive force (emf) of the whole cell reactioncalculated from the Nernst equation.

η_(c) =diffusion (concentration) or charge-transfer overpotential at thecathode surface. Absolute values are necessary since the cathodicover-potential is always negative.

η_(a) =diffusion (concentration) or charge-transfer overpotential at theanode surface.

IR=ohmic potential drop due to resistance to ionic mobility throughsolution.

Several important points should be noted here. First, the ohmicpotential drop is linear across the anode--cathode space (l) and isdependent on the superficial current density, i.e., the current densitywith respect to the projected area of the electrode face (A), not thespecific areas of the graphite particles themselves Thus, ##EQU1## wherek=specific conductivity (ohm cm)⁻¹ Within a porous bed, the ohmic termand in fact the entire solution potential becomes more complicated. Theconductivity also changes as the solution becomes depleted with thereaction zone, unless sufficient supporting electrolyte is present tomaintain a high level. In the absence of supporting electrolyte, thespecific conductivity of the standard 200 ppm solution was about 0.003(ohm cm)⁻¹. For the cell shown in FIG. 1, hydrogen evolution was notedat about 0.1 amp resulting in the following ohmic potential drop:##EQU2##

    V.sub.Ω =1.6 volts                                   (5)

Second, the anodic and cathodic overpotentials, in contrast to the ohmicdrop, depend on the current density with respect to individualparticles. The mathematical analysis of electrode potential, currentdensity and distribution in flow-through electrodes is complicated andhas been worked out only in the last few years.

Most commercial metal plating is done under conditions of diffusion(concentration) control where the diffusion overpotential is given by##EQU3## where R=gas constant (8.314 joule.°K.⁻¹)

T=absolute temperature (°K.)

n=number of electrons in half-cell reaction

F=Faraday constant (96,487 coulomb. mole⁻¹)

C_(s) =metal concentration at electrode surface (mole. cm⁻³)

C_(b) =metal concentration in bulk solution (mole. cm⁻³)

For metallic reduction reactions, η_(d) reaches a maximum of about 0.05volts at which point its limiting current density, il, is reached, andthe metal is being reduced as fast as it possibly can. ##EQU4## whereD=diffusivity (˜0.001-0.005 cm². sec⁻¹)

δ=diffusion layer thickness (˜0.05 cm)

When the voltage limit is exceeded, hydrogen gas also begins to evolvewhose overpotential is typically in the range 0.5-1.0 volts, about thesame as that of oxygen evolution at the anode. Since il is directlyproportional to C_(b), it is evident that in very dilute solutions thelimiting current density is small so that in order to reach effluents <1ppm it may be necessary to "overkill" the system with current, thusdriving up the cell voltage, evolving much hydrogen and reducing currentefficiency.

Third, it should be noted the φ_(cell) is the minimum voltage requiredfor a reaction to go. It is the sum of the two half-cell reactionscalculated from the Nernst Equation and is based entirely onthermodynamics and is therefore current independent. A list of thestandard electrode potentials, φ_(cell), for the various half cellreactions observed in this study are useful for putting into perspectivethe reactions which are thermodynamically favored over others. Table IXis given in terms of reduction potentials which correspond to unitactivity of the ions concerned, but are often, as a rough approximation,equated to potentials for unit concentration. The sign convention issuch that a positive potential indicates that the reaction isthermodynamically spontaneous.

                  TABLE IX                                                        ______________________________________                                        Standard Electrode Reduction Potentials                                                                   φ°                                     Electrode Reaction          (volts)                                           ______________________________________                                        Cr.sup.+2 + 2e- → Cr -0.91                                             Zn.sup.+2 + 2e- → Zn -0.763                                            Cr.sup.+3 + 3e- → Cr 0.74                                              PbCrO.sub.4 + 2e- → Pb + CrO.sub.4.sup.-2                                                          -0.499                                            Cr.sup.+3 + e- → Cr.sup.+2                                                                         -0.41                                             Ni.sup.+2 + 2e- → Ni -0.250                                            CrO.sub.4.sup.-2 + 4H.sub.2 O + 3e- → Cr(OH).sub.3 + 50H-(pH                                       -0.13                                             Pb.sup.+2 + 2e- → Pb -0.126                                            2H.sup.+ + 2e- → H.sub.2                                                                           +0.000                                            Hg.sub.2 Cl.sub.2 + 2e- → 2Hg + 2 Cl- (Satd KCl)                                                   +0.245                                            Cu.sup.+2 + 2e- → Cu +0.337                                            O.sub.2 + 4H.sup.+ + 4e- → 2H.sub.2 O                                                              +1.229                                            Cr.sub.2 O.sub.7.sup.-2 + 14H.sup.+ + 6e- → 2Cr.sup. +3 + 7H.sub.2     O (pH 1-6)                  +1.33                                             HCrO.sub.4.sup.- + 7H.sup.+ + 3e- → Cr.sup.+3 + 4H.sub.2 O                                         +1.35                                             ______________________________________                                    

Three cases will now be considered to show the range over which theminimum voltage requirement can vary for certain whole-cell reactions inwastewater systems. In actual experiments, the ions were not at unitactivity but at 3-4×10⁻³ M concentration. By Nerstian calculations themagnitude of overall cell potential (or minimum potential which must beovercome) would be reduced by amount 0.06 volts.

Case I: Thermodynamically Most Favorable Case

    ______________________________________                                                                 φ°                                        ______________________________________                                        Cathode                                                                              Cu+2 + 2e- → Cu  +0.337                                         Anode  CrO.sub.4.sup.+2 + Pb → PbCrO.sub.4 + 2e-                                                      +0.499                                         Cell   Cu.sup.+2 + CrO.sub.4.sup.-2 + Pb → Cu + PbCrO.sub.4                                           +0.836                                         ______________________________________                                    

The large positive potential indicates that, barring kineticrestrictions, the reaction is spontaneous and will proceed by itself.However, to keep the reaction going at a desirable fast rate, i.e., tosurmount the overpotentials which increase with current, an additionpotential may need to be applied.

Case II: Intermediate Case

    ______________________________________                                                                   φ°                                      ______________________________________                                        Cathode  2 Cu.sup.+2 + 4e- → 2Cu                                                                        +0.337                                       Anode    2H.sub.2 O → O.sub.2 + 4H.sup.+ + 4e-                                                          -1.229                                       Cell     2Cu.sup.+2 + 2H.sub.2 O → 2Cu + O.sub.2                                                        -0.892up.+                                   ______________________________________                                    

In comparing Case I and Case II it is apparent that the changed anodichalf-cell reaction has made the difference between a reaction which wasspontaneous and a reaction which must be driven by about 0.9 volts.

Case III: Thermodynamically Least Favorable Case

    ______________________________________                                                                   φ°                                      ______________________________________                                        Cathode  2 Zn.sup.+2 + 4e- → 2 Zn                                                                       -0.763                                       Anode    2H.sub.2 O → O.sub.2 + 4H.sup.+ + 4e-                                                          -1.229                                       Cell     2 Zn.sup.+2 + 2H.sub.2 O → 2Zn + O.sub.2                                                       -1.992up.+                                   ______________________________________                                    

In commercial zinc electrowinning, the cell voltage is about 3.5 voltsshowing that in addition to the 2-volt minimum, another 1.5 volts inoverpotentials are inherent in the operation and must also be overcome.

In a flow-through system, not only must the voltage requirements be met,but also a sufficient current must be available to remove all metallicions before they leave the reaction zone or zones if multiple stages areused. The theoretical current required for total metals removal may beeasily calculated from Faraday's Law: ##EQU5## In a batch or closedsystem, this equation relates the mass, m, of an element of atomicweight, M, which is reacted at an electrode by Δz electrons to thepassage of current, I, through the solution for time, t. In a continuousor flow-through system, the mass per unit time is equivalent to aconcentration, C, times a flow rate, q, and the theoretical currentbecomes ##EQU6## where q [=] cm³ min⁻¹

C[=] g·l⁻¹

I [=] amp=coulomb. sec⁻¹

Theoretical currents assume 100% current efficiency, i.e., noredissolution of metal, no side reactions such as hydrogen evolution,and no non-electrochemical reaction which removes the ion from solutionlike adsorption or hydrolysis.

It has been shown that in multi-component systems, the currentdistribution is such that the more noble metal reacts at the front endof the porous electrode while the more active metal reacts at the backend. Thus efficient use may be made of a properly designed electrodeeven if it is relatively thick (1-2") if several reaction zones arepresent within it. Furthermore, since reactions are highly sensitive tovariations in flow rate, a thick electrode would tend to confinereactions to discrete zones by minimizing hydrodynamic fluctuations likechanneling.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

I claim:
 1. A cell for removing chromium ions from waste streamscomprising:a. a container for holding the waste stream to be treated: b.an inlet tube into said container for introducing the waste stream intosaid container; c. an outlet tube in said container for delivering atreated waste stream from said container; d. a packed bed of lead shotforming an anode near the inlet of said container; e. a cathode ofconductive carbonaceous particles near the outlet of said container; f.means for applying positive voltage to the bed of lead shot; g. meansfor applying negative voltage to the bed of conductive carbonaceousparticles; and h. a trap for collecting lead chromate which sloughs offthe anode as chromate ions react with the lead shot when the stream tobe treated is flowed through the cell and voltage is applied to thecathode and anode.
 2. The cell as set forth in claim 1 wherein the trapis located below the beds forming the cathode and the anode.
 3. The cellas set forth in claim 2 wherein the anode is formed by a currentcollector forming one support wall for the lead shot and a screen formaintaining the packed bed of lead shot.
 4. The cell as set forth inclaim 2 wherein the cathode is formed by a current collector forming onesupport wall for the carbonaceous particles and a screen for maintainingthe bed of carbonaceous particles.
 5. A cell for removing chromium ionsfrom waste streams comprising:a. an inner cylinder formed of a porousmaterial which permits flow of electrolyte but prevents the passage ofsaid particles; b. a current collector positioned in said inner cylinderconnected to a source of positive voltage; c. a column of lead shotwithin said inner cylinder; d. an outer cylinder surrounding said innercylinder for containing a waste stream to be treated; e. a bed ofconductive carbonaceous material between the outside wall of said innercylinder and the inside wall of said outer cylinder; f. a currentcollector connected to a source of negative voltage in electricalcontact with said conductive carbonaceous particles; g. an inlet forfeeding waste streams into said cell located at the top of said innercylinder; h. an outlet for removing effluent from said cell located atthe bottom of said outer cylinder between the outside wall of said innercylinder and the inside wall of said outer cylinder; and i. a trap forcollecting lead chromate located beneath the bed of lead shot, said bedof lead shot in said trap being separated by a grid which prevents thepassage of lead shot but permits the passage of lead chromate.
 6. Thecell as set forth in claim 5 including a means for circulating wastestreams from which chromium ions have been removed from said innercylinder to the space between said inner cylinder and said outercylinder.